The invention describes a burner for a heat producer according to the preamble of claim 1.
Premix burners are known from EP 0 321 809, EP 0 780 629, WO 9317279, as well as EP 0 945 677; in them, a combustion air stream is introduced tangentially into the interior of a burner by means of a swirl producer, and is mixed with fuel. At the burner outlet, the vortex flow which arises bursts open at a sudden change of cross section, with the initiation of a back-flow zone which serves to stabilize a flame in the operation of the burner.
Although such burners make possible an operation with very low pollutant emissions, they often operate dangerously near to the extinction limit of the flame. Flame temperatures usually obtained with the lean premix flames of such burners are about 1,700-1,750 K. The extinction limit of the flame is given as about 1,650 K. This value is comparatively high. This is based on the low fuel ratio in the fuel-air mixture. This reduces the flame speed, which in turn results in a flame front which is widely expanded spatially and is hence unstable.
A stronger enrichment of the mixture would, however, drive the pollutant emissions higher and would negate the value of the use of lean premix burners.
The present invention has as its object to improve the stability of the lean premix combustion of modern burners of the kind mentioned at the beginning, as particularly used in the combustors of gas turbines, in that the distance between the flame temperature and the extinction limit temperature is enlarged. Here an essential raising of the combustion temperature is to be avoided in order to furthermore ensure a low-pollutant operation.
This is attained according to the invention in that the burner has a combustion gas mixing section at a downstream end, the said combustion gas mixing section partially projecting into a combustion space, and having combustion gas inlet apertures into the combustion space upstream of its mouth, through which combustion gas inlet apertures an amount of combustion gas flows from the combustion space into the combustion gas mixing section.
The invention makes use of the knowledge that an increase of the temperature of the fresh gasxe2x80x94and thus of the fuel-air mixturexe2x80x94results in an increase of the flame speed. In the relevant region, an increase of the fresh gas temperature by 300 K leads to about a doubling of the flame speed. As a result, the extent of the flame front is reduced, and the extinction temperature of the burner falls.
The core of the invention is thus an increase of the temperature of the fuel-air mixture in the process of combustion. A preheating of the combustion air is actually no longer realizable, precisely in gas turbine applications. Therefore, according to the invention, a combustion gas mixing section projecting into the combustion zone is used, in which on the one hand the premixed fuel/air mixture flows in as fresh gas, but in which on the other hand hot combustion gases flow in from the combustion space into the combustion gas mixing section in an upstream region of the mixing section, mix with the fresh gas in the mixing section, and thus raise the temperature of the combustion zone forming downstream of the combustion gas mixing section.
As described above, the extinction limit temperature of the flame is thereby lowered, and thus the flame stability is improved at the same combustion temperature.
It is indeed in the foreground that raising the mixture temperature increases the combustion temperature and thus the formation of nitrogen oxides; however, it should not remain unconsidered that the fuel air mixture is mixed with inert combustion gas. Hence the middle flame temperature is raised, but the power density and the temperature rise decrease, which compensates for the effect on the pollutant formation and particularly the formation of nitrogen oxides. The effects combine favorably when the mass flow of the admixed combustion gases is between 5% and 60% of the air mass flow supplied.
The admixture of combustion gases can be supported by suitable constructional measures. In particular, the axial flow cross section of the mixing section can be shaped so that at the place at which the combustion gas inlet apertures are located, a reduced pressure predominates relative to the combustion space. This can be attained, for example, in that the axial flow cross section has a sudden cross section widening, at which an eddy with a reduced pressure forms. The combustion gas inlet apertures are in this case arranged immediately downstream of the sudden change in cross section. In operation, the combustion gases are sucked into the eddy. Care has to be taken here that the cross section ratio of the flow sections upstream and downstream of the change in cross section is not too large, so that the swirl flow produced in the burner is maintained as far as the mouth of the mixing section in the combustion space, which is essential for the function of the burner mentioned in the preamble of the claims. A cross-sectional surface ratio in the region of 1.05-2.5 ensures good operating performance.
A further possibility of affecting the pressure ratios toward a strengthened mixing-in of combustion gases, by means of the pressure ratios in the combustion gas mixing section, is represented by a diffusor-like formation of the mixing section downstream of the combustion gas inlet apertures; also, a convergent-divergent course of the mixing section, in which the combustion gas inlet apertures are situated in the region of the narrowest flow cross section, is possible. The diffusor half-angle of the divergent portion of the combustion gas mixing section is in these cases to be in the region of 3xc2x0 to 10xc2x0, preferably 5xc2x0.
The invention is based on premix burners which are well known and familiar to the person skilled in the art from the prior art cited at the beginning. The invention can be directly combined with all the familiar kinds of swirl producers and burners disclosed in the documents cited there and developed from these documents and known per se to the person skilled in the art, and as only incompletely reflected in the preferred variants given in the dependent claims, among the multiplicity of possible embodiments.
The wall of the combustion gas mixing section is situated in operation in a strong hot gas exposure. In particular, when using conventional materials, it is advantageously embodied as cooled. A film cooling is preferred for reasons of cooling efficiency.
On the other hand, it is possible to mechanically decouple the combustion gas mixing section from the rest of the burner components, that is, from the swirl producer and/or a mixing tube which may possibly follow the swirl producer. This advantageously facilitates the use of materials whose expansion coefficients and thermal resistance are greatly different from those of the burner. Since the combustion gas mixing section furthermore has no appreciable mechanical loads to carry, it can advantageously be completely embodied in ceramic. In this case, cooling can be omitted in spite of the hot gas exposure of the mixing section, or the cooling can be a closed embodiment. Abstaining in this manner from blowing cooling medium into the region of the flame immediately confers advantages which are recognized by a person skilled in the art.